Nutrient pattern driven by water-sediment regulation favors cyanobacteria in yellow river ecosystems

Large-scale river management schemes are critical for flood control, but their ecological consequences are often poorly understood. The Yellow River's Water-Sediment Regulation scheme (WSR), an annual engineered flood pulse designed to preserve reservoir capacity and scour the downstream channel, represents a strong yet poorly understood ecosystem disturbance. Here, we combined multi-year high-frequency (2022-2024) monitoring data with high-frequency field sampling and ¹⁵N isotope tracer experiments to reveal the mechanistic pathway linking sediment disturbance to phytoplankton regime shifts. Our results indicate that the WSR was associated with a rapid succession from Bacillariophyta to Cyanobacteria dominance. This ecological shift was closely linked to a fundamental reconfiguration of the riverine nutrient regime, characterized by a sharp decrease in the total nitrogen to phosphorus (TN/TP) ratio and an approximately 12.7-fold increase in the ammonium-to-nitrate ratio. Mechanistically, isotope tracing experiments at a key confluence site revealed that high suspended sediment concentrations stimulated all major nitrogen transformation pathways, including denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA), leading to an overall decrease in dissolved TN. However, the increase in DNRA was disproportionately large (a 31-fold increase), causing it to become the main pathway (53.3%) and resulting in a net accumulation of ammonium, which fundamentally altered nitrogen speciation. Our findings uncover a critical trade-off between the engineering objectives of sediment management and the ecological imperative of maintaining water quality. This work provides a new predictive framework for assessing the impacts of pulsed hydrological disturbances on riverine ecosystems globally, highlighting the urgent need for integrated management strategies that account for these powerful biogeochemical feedbacks.